Process for the production of radioactive substances



1 2, 194%. E. FERMI ET AL 26,634 PROCESS FOR THE PRODUCTION OFRADIOACTIVE SUBSTANCES Filed oc-t. s, 19:55

I NVENT 0 RS Patented July 2, 1940 UNITED STATES PROCESS FOR- THEPRODUCTION OF RADIOACTIVE SUBSTANCES Enrico Fermi, Edoardo Amaldi, BrunoPontccorvo, Franco Rasetti, and Emilio Segre, Rome,

Italy, assignors to New York, N. Y.,

G. M. Giannini & 00., Inc., a corporation of New York ApplicationOctober 3, 1935, Serial No. 43,462

, In Italy October 26, 1 934 7 Claims.

This invention relates to the production of isotopes of elements fromother isotopes of the same or difierent elements by reaction withneutrons, and especially to the production of artificial radio activityby formation of unstable isotopes.

It has been known for many years that, although each chemical elementhas always the same atomic number or charge, it may exist in differentforms having different atomic weights. These forms of the elements arereferred to as isotopes.

It has also been known that the radio-active elements, by disintegrationor break down occurring in their nuclei are spontaneously converted intovarious isotopes of other elements. Thus, for example, the radio-activeelement uranium may be converted into lead of atomic weight 206, whilethe element thorium may be converted into a different isotope of atomicweight 208.

It has long been known that such spontaneous disintegration ofradio-active elements is accompanied by emission of alpha, beta, andgamma rays, that is to say, of the helium nucleus, electrons, andelectromagnetic radiation of extremely short wave length.

In more recent years it has been demonstrated that isotopes of variouselements could be converted into other isotopes of the same or difierentelements by bombardment with alpha particles, diplons, protons or gammarays of very high energy, and that the isotopes thus produced may beunstable so as to decompose with features similar to those of thenaturally radio-active bodies. That is to say, radio-active isotopesmay, in this way, be artificially produced.

Accordingly, it is an object of the present invention to provide amethod and apparatus by which nuclear reactions can be carried on withhigh efficiency and with the heavier as well as with the lighterelements. A more specific object of the invention is to provide a methodand apparatus for artificially producing radio-active substances withefiiciency such that their cost may be brought below that of naturalradio-active materials.

Our invention is based upon the use of neutrons instead of chargedparticles for the bombardment and transformation of the isotopes.

All of the prior work on nuclear reactions has been done with highenergy particles and every effort has been bent toward increasing theenergy of the particles as the means of extending and making moreefficient the nuclear reactions. We have now discovered that effort inthis direption is sound only when charged particles are used whichrequire tremendous energy to break through the potential barriersurrounding the nucleus; and that if, instead of charged particles,neutrons are used for the nuclear reactions, the greatest efficienciesare in some cases attained with low energy or slow neutrons, e. g., ofthe order of a few hundred electron volts, or even much less down to asmall fraction of an electron volt.

Neutrons when produced in any ordinary manner, e. g., by the action ofradon on beryllium or of polonium on beryllium or by bombardment ofatomic nuclei with artificially accelerated particles, might have a verywide range of energies but high average energy. These energies range upto several million volts. It is necessary, therefore, if the greatestefficiency of reaction is to be attained, to reduce by artificial meansthe energy of these neutrons. We describe below a method for slowingdown fast neutrons.

We have demonstrated that the absorption of slow neutrons is anomalouslylarge as compared with that of the faster or higher energy neutrons. Thesimplest explanation for most cases is to admit that the neutron iscaptured by the nucleus with formation of an isotope heavier by one massunit. If this heavier isotope is unstable a strong inducedradio-activity may be expected. This occurs-for example, with silver andiridium which go over into radio-active isotopes. In other cases it isfound that no activation, or at least no strong activation, follows ananomalously large absorption- This is the case with many elements, e.g., yttrium and cadmium. In these cases the formation of a stablenucleus upon the capture of the neutrons is to be expected.

In some cases the absorption of the slow neutrons results in theemission of a relatively strong gamma-radiation with energycorresponding to the binding energy of the neutron. This gives areliable source of very hard gamma rays, even harder than the naturallyproduced gamma rays, e. g., from radium.

In view of these considerations it is obviously desirable to convert asmany as possible of the available neutrons into the slow or low energycondition in which they may be readily captured by the nuclei of thesubstance being reacted. We have found that it is possible to achievethe desired results by passing the neutron radiation against or througha screen of a suitable material.

The materials which have been found best suited to this purpose arethose containing hydrogen (including all its isotopes, but the lightisotope which predominates in natural occurrence being most eihcient)and especially water and the hydrocarbons, such as paraffin for example.Other materials, as for example beryllium, carbon, silicon, lead, showthis effect to a lesser degree. Other materials, of which iron is anexample, do not produce a similar effect to any practical extent,probably because of a relatively large absorption of the neutrons whentheir energy is reduced.

The increase in activity of the neutrons by such substances isapparently due to two eifects both resulting from collisions of theneutrons. In the first place it is readily shown that an impact of aneutron against a proton reduces, on the average, the neutron energy bya factor l/e. From this it follows that ten impacts reduce the energy toabout l of its original value. Assuming the initial energy to be 4-10electron volts, the energy after ten impacts would be about 200 electronvolts and 20 impacts would reduce the energy of the neutron down to avalue corresponding to thermal agitation. Thus the first importanteffect is probably the reduction of the energy of the faster neutrons byimpact and the efliclency of hydrogen for this purpose is probably dueto the low mass of the hydrogen nucleus. Although we refer to thenucleus, because almost the entire mass is represented by the nucleus,it will be understood that the impact for slowing down may be, and forreasons of economy ordinarily will be, with atoms, '1. e., combinednuclei.

The second probable effect is the scattering and reflection of theneutrons.

Hydrogen is so much more effective than any of the other elements forreducing the energy of neutrons that it will ordinarily be used. It mustnot be overlooked, however, that theelements having a lesser effectoffer possibilities for control of the neutron energy. Where neutrons ofinitially lower energy are used or where their use requires a higherenergy than in the re: "ti ms with which we are here especially concernethe less effective elements may be used singly or combined with elementsof different energy reducing power.

The density of the energy reducing or scattering substance has alsobeenfound to be an important factor. This follows, likewise, from thetheoretical explanation given above. If the energy reduction andscattering of the neutrons is due to impact with atomic nuclei, theprobable frequency of such impacts will be directly dependent upon thenumber of atoms in a given space. For this reason such substancesordinarily should be used in liquid or solid (i. e., nongaseous) formand, so far as is practicable, substances will be chosen having as highas possible a proportion of hydrogen in the molecule. Here again, thegaseous state under various pressures, and substances having lesshydrogen offer the possibility for accurate control if less than themaximum slowing of neutrons is desirable.

It will be readily understood from what has been said above that thegreatest effect is attained if the source of neutrons and the substancebeing irradiated are both surrounded by the energy reducing andscattering material. This could easily be accomplished in many cases byimmersing the neutron source in a solution or emulsion of the substancebeing bombarded. We have illustrated diagrammatically in Figures 1 and 2arrangements by which this may be accomplished.

In Figure 1 a cylindrical paraflin block I0 is provided with a hole llinto which is inserted a source of neutrons, e. g., a tube containingradon and beryllium. The material being irradiated is placed above thesource on the paraflin block as shown at I! and is covered by a secondparaffin block l3 having a central opening l4 to accommodate thematerial being irradiated. For the treatment of small amounts ofmaterials the block I may be, for example, about 24 centimeters indiameter and about 14 centimeters in height with the neutron sourceabout 2 centimeters under the upper surface. It will be observed thatthese dimensions give radial thickness of the material surrounding theneutron source approximately equal to the mean free path in thesubstance of the high energy neutrons.

Where the substances to be irradiated are soluble in or can be suspendedin water or a hydrocarbon or other energy reducing or dispersingsubstance, etc., may be formed and the substances irradiated therein byimmersing the neutron source directly into the solution etc. (See Figure2.)

The hydrogen which serves to reduce the energy of the neutrons may alsobe in chemical combination with the substance being irradiated.

Figure 2 is a diagrammatic illustration of a neutron irradiating devicein which the substance being irradiated is dissolved or dispersed in theenergy reducing or dispersing material. A suitable vessel 20 is providedfor holding the solution or dispersion 2| and into this is immersed theneutron source 22.

Instead of the radon beryllium source, any other source of neutrons maybe used, as for example neutron tubes as-developed by Oliphant and asmore recently developed by laboratories of the General Electric Companyand the Westinghouse Electric and Manufacturing Company or cyclatrons asdeveloped by Lawrence. In such tubes deuteron oxide (heavy water), whichmay be separated by known methods from naturally occurring water, isbombarded with deuterons accelerated in an electric field produced by agrid tube. The deuteron nucleus is disintegrated with the emission ofneutrons.

Obviously the apparatus can be endlessly modifled, the essential beingthe combination of the energy reducing substance near the substancebeing irradiated, and a suitable source of neutron radiation.

In the following we have tabulated the results of various irradiationswhich we have carried out:

1. Hydrogen-No activity could be detected either in water or in paramnirradiated in a large can of water with 500 millicuries Rn+Be forseveral days.

3. Lithium.-Lithium hydroxide was found to be inactive after irradiationwith slow neutrons (14 hours, 400 millicuries) Although lithium remainsinactive, it strongly absorbs the slow neutrons; half=value thickness6:6.05 gm./cm. This absorption is not accompanied by a 'y-radiation. Itwas shown independently by Chadwick and Goldhaber" and by us that whenthe slow Nature, vol. 135, p. 65 (1935).

neutrons are absorbed, heavy charged particles are emitted. According toChadwick and Goldhaber, the nuclear process is represented by thefollowing reaction,

4. Beryllium-Metallic beryllium (purity 99%), strongly irradiated withslow neutrons, showed only an extremely weak activity possibly due toimpurities. Owing to the very strong activation of several elements whenirradiated under water, impurities might easily be misleading.

5. Boron.--Metallic boron irradiated 14 hours under water with 500millicuries was found inactive. Boron has the highest absorptioncoefficient as yet found for slow neutrons, 6:0.004 gm./cm.corresponding to a cross-section of about 3.10" cm. No v-rays have beenfound to accompany this absorption: instead of a 'y-radiation in thiscase as well as for lithium, a-particles are emitted, as was shown byChadwick and Goldhaber (Nature, vol. 135, 1935, p. 65) and by us. Thiseffect can be easily detected by the strong discharge in an ionizationchamber filled with boron trifiuoride surrounded by paraflin andirradiated with a Po-l-Be neutron source. Screening the ionizationchamber with a thin cadmium foil in order to absorb the slow neutrons,reduces considerably the ionization current. The same effect wasobserved with the ionization chamber filled with air, some boron beingspread on its floor. The emission of (Ir-particles was also detectedwith a small ionization chamber connected to a linear amplifier, eitherspreading some boron on its walls or filling it with boron trifiuoride.In order to explain these phenomena we have proposed the nuclearreaction,

6. Carbon-No activity; see hydrogen.

7. Nitrogen.-Ammonium nitrate irradiated 12 hours with 600 millicuriesunder water showed no activity.

8. Oxygen-No activity; see hydrogen.

9. Fluorine-Both activities of this element (periods 9 seconds and 40seconds are not sensitive to hydrogen containing substances.

11. Sodium.-This element has two activities: one of these (period lseconds) is not sensitive to hydrogen-containing substances. A very weakactivity with a long period was reported by Bjerge and Westcott. As thisactivity is strongly enhanced by water, we were able to measure itsperiods with reasonable accuracy and found it to be 15 hours. Owing tothe theoretical importance of this activity, we compared very carefullyits decay curve with that of the long period of aluminium in order tocheck their identity. For a chemical investigation of the activesubstance we irradiated pure sodium carbonate. We dissolved theirradiated substance in hydrochloric acid and added aluminum andmagnesium chlorides. A precipitate of the hydroxides of the latterelements obtained by adding ammonia, was found inactive. Afterwards weadded some sodium fluoride to the solution, and precipitated thefluorine as barium fluoride; this precipitate was also found inactive.The solution containing the original sodium was then evaporated andignited gently, in order to eliminate neon, where an active isotope ofthis element would have been formed. The activity was found in the driedsodium salt. We conclude that the active product is an isotope ofsodium, Na. The same isotope was produced by us last year by bombardingmagnesium or aluminum with neutrons. *Na has also been produced recentlyin considerable amounts and studied very completely by Lawrencebombarding several elements with artificially aecelerated particles.

12. Magnesium-Pure magnesium oxide, especially tested by us in order toensure that it was aluminium free, was irradiated under water. Thesubstance was kept at some distance frpm the source in order to preventthe activation of the periods not sensitiveto water. A new very weakactivity with a period of about 10 minutes was found. As this periodcoincides with the 10- minute period of aluminum, which is known to bedue to Mg (see aluminium), it is very likely that it is due to the sameisotope formed by the capture of a neutron by Mg present in an amount of11% in ordinary magnesium.

The IO-second period is insensitive to water.

13. AZuminium.--Aluminium irradiated in water shows a'fairly strong newactivity decaying with a period of 2-3 minutes (measured with anionization chamber). Irradiated outside of water, this activity isextremely weak. As the period of the new activity coincides with the 2-minute period of silicon, which is due to Al, we assume that thisactivity is also due to the same isotope formed by capture of a neutronfrom AI.

The second period of aluminium has been.

measured with the ionization chamber and found to be 10 minutes insteadof 12. This activity is insensitive to water. A chemical separation ofthe carrier of this activity has been performed. Irradiated metallicaluminium was dissolved in a caustic soda solution and magnesiumchloride was added. The precipitate of magnesium hydroxide carried the10-minute activity. We assume that the active isotope is "Mg formedaccording to the reaction,

14. Silicon.-We have determined with the ionization chamber the shortperiod of this element, finding it to be 2-3 minutes. This activity isinsensitive to water.

Besides this activity, we found a new, longer period of some hours infused silica irradiated in water. This activity is very weak and verysensitive to water. We think probably that its carrier is Si which isobtained by irradiated phosphorus and has a period of 2-4 hours. Sicould be formed by capture of a neutron from Si present in an amount of3%.

15. Ph0sph0rus.-The short-period activity of this element (2-3 minutes)is not enhanced by water. Curie, Joliot and Preiswerk ascribe thisperiod to Al. A chemical test in favour of this hypothesis is thefollowing: we irradiated phosphoric acid, neutralized the solution withsodium carbonate, and added aluminium chloride; the activity was foundto be concentrated in the precipitated aluminium.

We have observed, with the aid of the ionization chamber, thedecay-curve of the longer period of phosphorus. Its period is 2-4 hoursinstead of 3 as given before. We have also measured, with the ionizationchamber, the half-value thickness of the corresponding p-rays and foundit to be 0-15 gm./cm. Al.

16. SuZphur.-We have determined in the ionization chamber the period ofphosphorus extracted from irradiated sulphur. We found: period 14 days,half-value thickness of the 3-rays 0/10 gm./cm. Al.

17. ChZorine.-Chlorine irradiated under water showed a new period of 35minutes measured electrometrically.

Chlorine absorbs fairly strongly the slow neutrons (half-value thickness6:0.3 gm./cm.

The process of absorption is accompanied by emission of -rays.

19. Potassium-We have found in irradiated potassium an induced activitystrongly sensitive to water, decaying with a period of 16 hours. Achemical investigation of the carrier of the activity, performed by thesame method described for sodium, excluded the elements Cl, A, Ca. Weconclude therefrom that the activity is probably carried by an isotopeof potassium. According to v. Hevesy, this isotope is to be identifiedwith a K, that was obtained by him by neutron bombardment of scandium,and has the same decay period.

20. Calcium.No activity was found in calcium fluoride irradiated 14hours in water with a 600 millicuries source. A very weak activitysensitive to water has been demonstrated. 23. Vanadium.-The decay of theactivity induced in vanadium has been measured in the ionization chamberwith the following results: half-value period 3-75 minutes; half-valuethickness of the p-rays 017 gm./cm. Al. The p-rays are accompanied by a'y-radiation. The activation of vanadium is strongly sensitive tohydro,- genated substances.

24. Chromium.--The insensitive to water.

25. Manganese.'1'he activity with short period (345 minutes) isinsensitive to water. On the other hand, the activity with longer period(2-5 hours measured in the ionization chamber) is strongly enhanced bywater. Half-value thickness of p-rays measured electrometrically is 0-14gin/cm. Al; the disintegration is accompanied by -rays. The 2-5-hourproduct is known to be an isotope of manganese. In order to get newevidence in favour of the fact that the active product is really anisotope of manganese, we first concentrated the activity obtained inirradiated manganese permanganate by a precipitation of maganesecarbonate. The carbonate containing the activity was then dissolved inhydrochloric acid, and large amounts of chromium, vanadium and ironsalts were added to the solution. Afterwards the manganese was separatedonce again as dioxide, with nitric acid and sodium chlorate. Themanganese precipitate carried the activity, while the fractionscontaining chromium, vanadium and iron were found to be inactive.

26. Iron-The activity of this element (period 2-5 hours) is insensitiveto water. Half-value thickness for the absorption of slow neutrons 8gm./cm.

27. Cobalt-This element absorbs fairly strongly the slow neutrons;half-value thickness 0-7 gin/cm}. The absorption is accompanied by theemission of a -radiation.

28. NickeL-Strongly irradiated nickel showed only a dubious trace ofactivity.

29. Copper.-Both induced activities of this element (periods 5 minutes,measured electrometrically, and 10 hours) are strongly enhanced bywater. Copper absorbs the slow neutrons with a half-value thickness ofabout 3 gm./cm. this absorption is accompanied by a weak -y-radiation.

Irradiated metallic copper was dissolved in hydrochloric acid, and smallquantities of cobalt, nickel and zinc salts were added. Copper sulphidewas precipitated from the acid solution and found to be active. Theprecipitates of the zinc, cobalt activity of chromium is and nickelsulphides, obtained by neutralizing the solution and adding ammoniumsulphide, were 4 sulphide in inactive. As the time employed for thistest was rather long, the test refers only to the longer period. Thecarrier of this activity can then be assumed to be an isotope of copper.

30. Zinc.The activity of the short period of zinc is not enhanced bywater. The longer period was measured electrometrically and found to be10 hours. The carrier of this activity has been investigated by means ofthe following test: irradiated metallic zinc was dissolved inhydrochloric acid, and a small quantity of copper. nickel'and cobaltsalts added. Copper was precipitated partially by reduction on smalltraces of undissolved metallic zinc and partially as ihez -acidsolution. The collected copper wa fiiongly active. Neutralizing thesolution andadding ammonium sulphide, the other elements wereprecipitated and found to be inactive.

31. Gallium.The 20-minute period (measured electrometrically) is notvery sensitive to water. Half-value thickness of the correspondingp-rays is 0.17 gm./cm. Al. The carrier of this activity is probably anisotope of gallium. In order to test this point, we irradiated galliumnitrate and afterwards added to the solution traces of copper and zinc.Copper was separated as a metallic deposit on zinc powder and zinc aszinc mercury sulphocyanate after adding mercury sulphocyanate. Bothelements were found to be inactive.

Besides this 20-minute activity, we have also found, irradiating underwater, a new activity which is accompanied by a rather strong'y-radiation; it decays with a period of 23 hours (measuredelectrometrically) 33. Arsenic.--'I'he activity of this element isstrongly sensitive to water. We have measured electrometrically itsperiod (26. hours) and its half-value thickness of the p-rays (0. 16gm./cm. Al).

34. Selenium.-The activity of this element (period 35 minutes) issensitive to water. Irradiated selenious anhydride was dissolved in 30%hydrochloric acid and some arsenious anhydride added to the solution. Weprecipitated metallic selenium by reduction with gaseous sulphurousanhydride, and found it strongly active. We precipitated metallicselenium by reduction with gaseous sulphurous anhydride, and found itstrongly active. We precipitated from the solution arsenic sulphide andfound it inactive. This test seems to rule out also germanium, and weconclude that the activity is due to an isotope of selenium. J

35. Bromine-Both 'activities of this element are sensitive to water. Theperiods have been measured electrometrically; they are 18 minutes and4-2 hours. The half-value thickness of the 'y-rays is for bothactivities 0-12 gm./cm. A1, and both are accompanied by 'y-rays.

38. Strontium. 0 activity was found after a long and strong irradiationunder water.

39. Yttrium.Strongly irradiated yttrium oxide showed only a very weakactivity possibly due to impurities. Yttrium absorbs very intensivelythe slow neutrons (half-value thickness 6:0.015 gmJcmF). This absorptionis accompanied by 'vays- .40. Zirconium.Strongly irradiated zirconiumnitrate showed only a very weak activity probably'due to impurities.

- 41. Ni0bium.-The same as zirconium.

43. Rhodium.-The short-period activity is sensitive to water. Period andhalf-value thickness of the p-rays have been determinedelectrometrically (44 seconds; 0-15 gm./cm. Al). We also made a moreaccurate measurement in the ionization chamber of the longer period andfound it to be 3-9 minutes. The activity is accompanied by a weak'y-radiation. Rhodium absorbs fairly strongly the slow neutrons(half-value thickness 0-3 gm./om. the absorption probably corresponds tothe formation of the active isotopes.

46. Palladium.-Also the activities of this element are sensitive towater. We find at least two periods: a short one of about a quarter ofan hour and one of about 12 hours.

4'7. Sz'Zver.--The two periods have been redetermined with theionization chamber. They are 22 seconds and 2-3 minutes. They are bothvery sensitive to Water. To the strong activation of this elementcorresponds a considerable absorption for slow neutrons (half-valuethickness 6:1-2 gm./cm.

We added palladium nitrate and rhodium chloride to a solution ofirradiated silver nitrate. Adding hydrochloric acid, we precipitatedsilver which was found active. From the filtered solution weprecipitated palladium with dimethylglioxime and rhodium by reduction.Both were inactive. This test is valid only for the longer period, owingto the time employed, and shows that the carrier of the activity isprobably an isotope of silver.

48. Cadmium-Cadmium irradiation under different conditions showedseveral weak activities with various periods not yet identified. Cadmiumabsorbs with great intensity the slow neutrons. (Half-value thickness0-013 gm./cm. The corresponding cross-section is the largest as yetfound for slow neutrons (a=10 cm. The absorption is accompanied by anintensive 'ydadiation and probably corresponds to the transformation ofa stable isotope of cadmium into another stable isotope of the sameelement.

49. Indium.-The activity induced in indium shows three periods: Theshortest period (13 secconds) has an activity highly sensitive to water.Also the second period (54 minutes, measured electrometrically) is verysensitive to water. Magnetic deflection experiments show that thecorresponding electrons are negative; their halfvalue thickness is 0-045gin/cm. A1. A still longer period of some hours is recorded by Szilardand Chalmers; this last activity is either insensitive to water or isonly moderately sensitive.

Chemical tests have been made in order to identify the carriers of thelast two activities. To a solution of irradiated indium nitrate, silverwas added and precipitated as silver chloride; the precipitate wasinactive. Afterwards we added to the solution tin, antimony and cadmiumand precipitated them as sulphides with sulphuretted hydrogen. Theacidity of the solution must be adjusted in such a way as to leave theindium in solution while precipitating the other metals. Thisprecipitate also was inactive; neutralizing the solution, weprecipitated the indium sulphide which carried the activity.

Corresponding to the strong activation of indium, it is found that thiselement has a considerable absorption power for the slow neutrons:half-value thickness 6:0-3 gm./cm.

50. Tin.-Tin strongly irradiated under water showed no activity.

51. Antimony.-.-We have found an induced activity in this element,decaying with a period of 2-5 days; the-activation is'sensitive tohydrogenated substances. The half-value thickness for the emitted p-raysis 0-09 gm./cm. Al. The following chemical test indicates that thecarrier of this activity is probably an isotope of antimony. Wedissolved metallic irradiated antimony in aqua regia and added some tinto the solution; after separation of tin as a sulphide according toClarke, we found the activity in a precipitate of sulphide of antimony.The antimony sulphide was then dissolved again; indium was added to thesolution and antimony separated as a sulphide in a moderately acidsolution; the solution was neutralized and indium precipitated and foundto be inactive. To a new solution of the antimony We added tellurium andiodine and separated the first by reduction and the second as a silveriodide. Both were inactive.

52. TeZZurium.Shows a weak activity sensitive to water; the periodresulted 45 minutes.

53. Iodinc. -Period and half-value thickness of the B-rays weredetermined electrometrically: period 25 minutes; half-value thickness0-11 gm./cm. Al. The activation is moderately sensitive to water.

56. Barnum-A new activity sensitive to water with a period of minuteshas been found. The following chemical test is in favour of theassumption that the carrier of this activity is an isotope of barium. Wedissolved irradiated barium hydroxide in hydrochloric acid, and added asmall quantity of sodium chloride and precipitated barium sulphate. Theactivity was carried by the precipitate; we evaporated the solution andfound the residual sodium to be inactive.

57. Lanthanum.-No activity was found after strong irradiation underwater.

58. Cerium.Same as lanthanum.

59. 'Praseodymium.-The short-period activity (5 minutes) is insensitiveto water. Irradiating under water we have found a new water-sensitiveactivity decaying with a period of 19 hours; half-value thickness of thecorresponding B-rays 0-12 gm./cm. Al (both measured electrometrically).

'72. Hafnium v. Hevesy has found an activity having a long period ofseveral months which is sensitive to water.

64. GadoZinium.--We irradiated under water a very pure sample ofgadolinium oxide. We found an activity, decaying with a period of 8hours.

'73. TantaZum.--Only a dubious activity was found after 12 hoursirradiation under water with 500 millicuries.

74. Tungsten.-Metallic tungsten was irradiated under water and showed anactivity decaying with a period of about 1 day.

We irradiated tungstic anhydride, dissolved it in caustic soda and thenadded and separated tantalum pentoxide which was found to be inactive.To the tungstic solution we added a nitric rhenium solution andprecipitated the tungstic anhydride adding hydrochloric acid. Theprecipitate carried the activity, while the rhenium, precipitated fromthe filtrate as sulphide, was inactive. As we have no hafnium, we havemade the following experiment in order to exclude an isotope of thiselement as carrier of the activity. From a solution of irradiatedtungstic anhydride in ammonia, we precipitated zirconium hydroxide. Theprecipitate was inactive. We conclude that the activity of tungsten isprobably due to an isotope of this element. L

75. Rhenium.-We irradiated. pure metallic rhenium under water; itsactivity is enhanced by water and decays with a period of about 20 tronsis 012 gm./cm. Al. The activity is probably carried by an isotope ofrhenium. Irradiated rhenium was dissolved in nitric acid; we addedtantalum and tungsten and separated them as tantalum pentoxide andtungstic anhydride. Both were inactive, while rhenium conserved theactivity.-

77. Iridium-The activity induced in this element is strongly sensitiveto water. Period and half-value thickness ofthe p-rays have beenmeasured in the ionization chamber; period 19 hours, half-valuethickness 0-12 gm./cm. Al. To the strong activation of iridiumcorresponds a strong absorption of-the slow neutrons; halfvaluethickness 0-3 gm./cm. the absorption is accompanied by the emission of'y-rays.

78. Platinum very pure metallic platinum 7 irradiated under water showedan activity decaying with a period of about 5-0 minutes.

79. G0ld.-The activity of this element is sensitive to water; itsperiodhas been measured electrometrlcally and is 2-7 days. The p-rays weremagnetically deflected and found to be negative. They have a very smallpenetrating power: half-value thickness 0-04 gm./cm. Al.

Strong -radiation is omitted during bombardf ment with slow neutrons.

80. Mercury.--No activity was found after strong irradiation. Thiselement absorbs intensely the slow neutrons, half-value thickness 0-2gm./cm. 'y-rays are emitted during the absorption.

81. Thallium.No activity was found after strong irradiation.

82. Lead.'1he same as thallium.

83. Bismuth-The same as thallium.

90. Thorium.-The l-minute and 24-minute (measured electrometrically)periods are scarcely sensitive to water.

92. Uranium.-We have also studied the influence of hydrogenatedsubstances on the induced activities of this element. (Periods 15seconds, 40 seconds, 13 minutes, 100 minutes.) The result was that whilethe activities corresponding to the first, third and fourth period areslightly increased by water, no increase was found for the activitycorresponding to the 40-second period.

Chemical evidence seems to indicate that the carriers of. the 13- andthe IOU-minute activities were not isotopes of any of the known heaviestelements, and that they were probably due to transuranic elements.

The precipitation of the activity with a sulphide was repeated,precipitating several metals (silver, copper, lead, mercury); theacidity of the solution (hydrochloric acid) was about 20%; sometimesslightly varied in order to facilitate the precipitation of the sulphideof the metal used. The yield in activity of the precipitate wasgenerally good-about 50%and varied according to the conditions of theprecipitation.

2,206,634 hours. The half-value thickness of the eleczirconium phosphatewas inactive. After the separation of zirconium we precipitated asulphide from the filtered solution, and collected the activity in thesulphide from the filtered solution, and collected the activity in thesulphide with the usual yield. According to von Grosse and Agruss, thisreaction must be considered a proof by the non-identity of the carrierof the activity with a protoactinium isotope. The 15-second, 13-minuteand IOU-minute activities are probably chain products, with atomicnumber 92, 93 and 94 respectively and atomic weight 239.

From the above tabulation it is apparent that the increase in activitiesby the hydrogen containing substances, etc., is particularly applicableto those nuclear reactions in which the neutron is captured with theformation of a heavier isotope of the same element; and the presentinvention makes possible numerous reactions of this type which could notbe appregiably carried out without the use of our invenion.

With these procedures the isotopes produced by the nuclear reactions areordinarily mixed with other substances, although in much higherconcentrations than was heretofore possible. We have utilized for theseparation of these isotopes and especially of the radio-active isotopesthe method of Szilard and Chalmers (Nature, vol. 134, page 462, 1934)and extended their procedure to cover other cases. examples andtheoretical discussion of our invention are set forth in ourpublications: Fermi, Amaldi, DAgostino, Rasetti, Segre, Proc. Roy Soc.,-A, vol. 146, p. 483 (1934); A, vol. 149, p. 522; Fermi, Amaldi,Pontecorvo, Rasetti, Ric. Scient.,"vol. 2, p. 380 (1934); Fermi,Pontecorvo, Rasetti, Ric. Scient., vol. 2, p. 380 (1934); Amaldi,D'Agostino, Segre, Ric. Scient., vol. 2, p. 381 (1934); Amaldi,DAgostino, Fermi, Pontecorvo, Rasetti, Segre, Ric. Scient., vol. 2, p.467 (1934); vol. 1, p. 123 (1935).

Claims directed to the broad method of producing radio-active substancesby a neutron reaction and to beta-emissive substances so produced arebeing presented in the'copending application Serial No. 57,325 to EnricoFermi, filed January 2, 1936.

Although we have herein described our invention in detail and specifiedparticular examples of apparatus and processes and various modificationsthereof, and have proposed various theoretical explanations, it is to beunderstood that these are not binding nor exhaustive but are intendedrather for the assistance of others skilled in the art to enable themmore easily to apply our invention under widely varying. conditionsencountered in actual practice and to change and modify the particularembodiments and examples herein described as may be necessary ordesirable under such varying conditions. The theoretical statements andexplanations are, of course, not conclusive and our invention'is in noway dependent upon their correctness. We have found them helpful andgive them for the aid of others, but our invention will be equallyuseful if it should prove that our theoretical conclusions are notaltogether correct.

We claim:

1. The process for the production of radioactive isotopes, whichcomprises generating neutrons having a high average electron voltage,slowing down and scattering said neutrons by projecting them through ascreen of an element of the class consisting of hydrogen, helium,beryllium, carbon, silicon and lead which screen is of such thicknessthat the neutrons are slowed down to an average energy of not more thana few hundred electron volts, then passing said neutrons into a mass ofan element of the groups having atomic numbers 11, 12, 13, 14, 17, 19,23, 25, 29, 31, 33, 34, 35, 43, 47, 48, 49, 51, 52, 53, 56, 64, 72, 74,'75, 77, 78, 79 and 92, and thereby producing from the latter element aradio-active isotope capable of emitting beta rays.

2. The process of producing radio-active isotopes which comprisesgenerating neutrons having a high average energy, slowing down saidneutrons so that they have an average energy of not more than a fewhundred electron volts by projecting them' through an energy-reducingscreen of an element of the class consisting of hydrogen, helium,beryllium, carbon, silicon and lead, and passing said neutrons ofreduced energy into a mass of ,an element of the group having atomicnumbers 11, 12, 13, 14, 17, 19, 23, 25, 29, 31, 33, 34, 35, 43, 47, 48,49, 51, 52, 53, 56, 64, '72, 74, 75, 77, '78,-79 and 92 to therebyproduce from the latter element a radio-active isotope capable ofemitting beta rays.

3. The process of producing radio-active isotopes which comprises,generating neutrons having a high average energy, slowing down saidneutrons so that they have an average energy of not more than a fewhundred electron volts by projecting them through a hydrocarbon, andpassing said neutrons of reducedenergy into a mass of an element of thegroup having atomic numbers 11, 12, 13, 14, 17, 19, 23, 25, 29, 31, 33,34, 35, 43, 4'7, 48, 49, 51, 52, 53, 56:64, 72, 74, 75, '77, 78, '79 and92 to thereby produce from said latter element a radio-active isotopecapable of emitting beta rays.

4. The process of producing radio-active isotopes which comprises,generating neutrons having a high average energy, slowing down saidneutrons so that they have an average energy of not more than a fewhundred electron volts by projecting them through paramn, and passingsaid neutrons of reduced energy into a mass of an element of the grouphaving atomic numbers 11, 12, 13, 14, 1'7, 19, 23, 25, 29, 31, 33, 34,35, 43, 47, 48, 49, 51, 52, 53, 56, 64, 72, 74, 75, 7'7, 78, 79 and 92to thereby produce from the latter element a radio-active isotopecapable of emitting beta rays.

5. The process of producing radio-active isotopes which comprises,generating neutrons having a high average energy, slowing down saidneutrons so that they have an average energy of not more than a fewhundred electron volts by projecting them through water, and passingsaid neutrons of reduced energy into a mass of an element of the grouphaving atomic numbers 11, 12, 13, 14, 1'7, 19, 23, 25, 29, 31, 33, 34,35, 43, 47, 48, 49, 51, 52, 53, 56, 64, 72, '74, '75, '77, '78, '79 and92 to thereby produce from the latter element a radio-active isotopecapable of emitting beta rays.

6. The process of producing a radio-active isotope from an element ofthe group having atomic numbers 11, 12, 13, 14, 17, 19, 23, 25, 29, 31,33, 34, 35, 43, 47, 48, 49, 51, 52, 53, 56, 64, 72, 74, 75, 77, 78, 79and 92, comprising the steps distributing the element throughout anenergy-reducing screen of an element of the class consisting ofhydrogen, helium, beryllium, carbon, silicon and lead, generatingneutrons having a high average energy, and projecting the high-energyneutrons through said energy-reducing screen to slow down the neutronsto an average energy of not more than a few hundred electron volts andthereby produce from said first element by the reaction of the neutronsof reduced energy a radioactive isotope capable of emitting beta rays.

7. The process of producing a radio-active isotope from an element ofthe group having atomic numbers 11, 12, 13, 14, 17, 19, 23, 25, 29, 31,33, 34, 35, 43, 47, 48, 49, 51, 52, 53, 56, 64, 72, 74, 75, 77, '78, 79and 92, comprising the steps distributing the element throughout anenergy-reducing screen of an element of the class consisting ofhydrogen, helium, beryllium, carbon, silicon and lead, generating at apoint substantially surrounded by said screen neutrons having a highaverage energy, and projecting the high-energy neutrons through saidenergy-reducing screen to slow down the neutrons to an average energy ofnot more than a few hundred electron volts and thereby produce from saidfirst element by the reaction of the neutrons of reduced energy aradio-active isotope capable of emitting beta rays.

EMILIO SEGRE. ENRICO FERMI. EDOARDO AMALDI. BRUNO PONTECORVO. FRANCORASETII.

